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I've been fighting with noise in my render. For draft renders I clamp the AO bounces in simplify, which clobbers the noise, but also the shadows. And I don't like that. So for a final render I want to turn that off. However the noise sky rockets, as you'd expect. Then I hit on the idea of removing the glass from my windows, and the noise almost vanishes.

Here's the render with glass in the windows:

enter image description here

And here it is with no glass in the windows:

enter image description here

It's not only less noisy, but brighter too. Both these are done at 128 samples with denoising intentionally turned off.

So how can I have glass in the windows and not cause the dimming of the light and the increase in noise? And why should glass cause such a massive difference?

My glass is simply a cube with this texture:

enter image description here

And yes, there's a portal in the window.

Majenko
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  • Do you have reflective and refractive caustics enabled or disabled? – Brenticus Feb 25 '19 at 16:00
  • At the moment both are enabled. With it disabled it goes almost black. I could crank up the HDR brightness, but the noise is still as bad as ever. If not worse. – Majenko Feb 25 '19 at 16:01
  • What about just disabling refractive? – Brenticus Feb 25 '19 at 16:02
  • Disabling refractive caustics only gives me very dark and noise. Disabling reflective only gives me a dim noisy image similar to the first image above. – Majenko Feb 25 '19 at 16:04
  • How thick is the window? – Brenticus Feb 25 '19 at 16:06
  • Ok. You can ignore the brightness problem. That's me clamping the indirect light. The glass massively increases the noise though. – Majenko Feb 25 '19 at 16:06
  • Glass just makes it harder for cycles to compute. Any ray that goes through the window now has at least 2 more deflections as it moves between materials. If you can upload the blend we can see what exactly is going on. There's a number of possible sources. – Brenticus Feb 25 '19 at 16:08
  • So I just have to suffer higher sample rates to offset the glass? BTW, the panes are 1.75 thick. In the scale I'm working in, that's 1.75cm. Double glazing thickness (but only one single cube not two). – Majenko Feb 25 '19 at 16:11
  • I have just been pointed to this by someone else. Implementing that transparent mixing with shadow / reflection ray setup certainly helps a bit. – Majenko Feb 25 '19 at 16:21
  • Disabling shadow ray visibility for the glass objects cures the problem entirely. Would that be a good solution, or iffy at best? – Majenko Feb 25 '19 at 16:27
  • https://blender.stackexchange.com/questions/2558/how-to-illuminate-the-darkness-inside-glass-objects-in-cycles – Duarte Farrajota Ramos Feb 25 '19 at 16:28
  • Possible duplicate of How to avoid noisy renders in Cycles? – Duarte Farrajota Ramos Feb 25 '19 at 16:28
  • https://blender.stackexchange.com/questions/1703/how-to-reduce-fireflies-in-cycles – Duarte Farrajota Ramos Feb 25 '19 at 16:28
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    Don't use the principled shader for the window. Use a modified glass shader Read: https://blender.stackexchange.com/questions/47851/how-can-i-make-a-more-realistic-glass-shader –  Feb 25 '19 at 17:46
  • Read also: https://blender.stackexchange.com/questions/108735/rendering-glass-layers-resulting-in-strange-dark-patches-in-final-but-not-pre/109003#109003 –  Feb 25 '19 at 17:55

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